1. Field of the Invention
The invention relates to an image forming apparatus using an electrophotographic process, an electrostatic recording process or the like, and particularly to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus, or a compound machine having a plurality of these functions.
2. Related Background Art
In conventional image forming apparatuses of an electrophotographic type, and above all, particularly a multi-color image forming apparatus for effecting color image forming, there has been widely utilized two-component development using a mixture of a nonmagnetic toner and a magnetic carrier as a developer.
The two-component development, as compared with presently proposed other developing processes, has such merits as the stability of image and the durability of apparatus, while on the other hand, only the toner is consumed with image forming and therefore, in conformity therewith, it is necessary to suitably supply the toner and control toner density (the weight ratio of the toner to the weight of the whole developer) within an appropriate range. It is a very important factor in stabilize the quality of image to control the toner density within an appropriate range, and various methods have heretofore been proposed and put into practical use.
For example, there have heretofore been proposed and practiced a method of directly detecting toner density in a developing device by the use of an optical sensor, a method of directly detecting toner density in a developing device by the use of an inductance sensor, a method of detecting the density of a patch image by an optical sensor to thereby indirectly detect toner density in a developing device (hereinafter referred to as the patch method), a method of indirectly detecting toner density in a developing device by the use of a video counter for integrating an image information signal (hereinafter referred to as the video count method), etc. The amount of toner to be supplied to the developing device is determined on the basis of the toner density detected by one of these methods.
Among them, the patch method is a method of reading the density of a reference toner image (hereinafter referred to as the “patch image”) formed on a photosensitive member as an image bearing member by a light source provided at a location opposed to the surface thereof and a sensor for receiving the reflected light thereof, and controlly the toner supply amount on the basis of the output value thereof to thereby effect toner density control. This method is widely used because when in a multi-color image forming apparatus, a plurality of developing apparatuses are provided around the photosensitive member, it is not necessary to provide a sensor for each developing apparatus and this is advantageous in terms of cost.
Also, when the patch detecting method is adopted for the toner density control of a two-component developer, it is desirable to widen the interval at which the patch images are formed to the utmost, from the point that consumption of excess toner becomes necessary to form the patch images, and the point that the ordinary image forming operation must be discontinued during the formation of the patch images and this becomes a factor which reduces productivity.
So, use is made of a method as described in Japanese Patent Application Laid-Open No. H5-27598 wherein the video count method is used in addition to the patch method. The video count method is a method of foreseeing the toner consumption amount from the integrated value of the video count number of the image density of an image information signal read by a CCD or the like, and effecting an amount of toner supply corresponding thereto, and the toner supply amount is calculated and supplied during each image forming operation, and this leads to the advantage that when a great amount of toner is consumed as when forming an image of high density, the toner supply is controlled so as to quickly provide proper developer density. On the other hand, when there is a difference between the toner consumption amount foreseen from the video count number and the actual toner consumption amount, there is the possibility that the toner density in the developer gradually deviates from a proper range.
So, use has heretofore been made of a combined method of normally effecting toner density control by the video count method, and effecting toner density control by the patch detecting method when the image forming operation has reached a predetermined number of times.
This combined method, as compared with the controlling method using only the patch detecting method, can greatly widen the interval between the patch detecting operations, and also can correct the deviation of the toner consumption amount by the video count method, by the patch detecting method and can therefore be said to be a very excellent method.
Now, when the image forming apparatus continues to be used for a long period of time, the toner in a toner storing portion storing therein the toner to be supplied becomes exhausted and therefore, a design is made such that the presence or absence of the toner remaining in this toner storing portion is judged to thereby call upon a user to supply a fresh toner. When this toner storing portion is made into a cartridge type detachably attachable to the image forming apparatus, a design should preferably be made such that if the toner in the toner storing portion is judged to be substantially absent, the user is called upon to interchange the toner storing portion made into a cartridge.
As this residual toner amount detecting means, there have heretofore been proposed and carried out a method using a piezosensor, a method using an antenna sensor, a method using an optical sensor, etc.
As another method, there is a method serving as toner presence or absence detecting means and toner density controlling means by the use of the output of a sensor for detecting the above-described patch image. Specifically, when the detection output of the patch image is a predetermined value or less or when an output of a predetermined value or less continues a plurality of times, the absence of the toner is judged. This method can perform both of toner presence or absence detection and toner density control by a single sensor and therefore, does not require the provision of a sensor exclusively for toner presence or absence detection, and is a very excellent method in terms of cost.
This method, however, performs toner presence or absence detection and toner density control by a single sensor and therefore, simply by looking at the detection output of the patch image, it is difficult to judge whether the toner is truly absent or the density of the patch image has become low due to any other factor than the toner density.
To enhance the accuracy of this toner presence or absence judgment, a threshold value to be compared with the sensor output detecting the patch image density when the absence of the toner is judged can be set to a considerably low value, but if this is done, the ordinary image density will be greatly lowered in the vicinity of the absence of the toner, and this is not preferable in practical use. Conversely, if the threshold value is set to a higher value, the lowering of the density can be suppressed, but the risk of misjudging the presence or absence of the toner will heighten.